This invention relates to abrasion-resistant thermoplastic articles and a method for their preparation. More particularly, it relates to the use of an improved primer in the fabrication of such articles.
Transparent plastics are widely used, particularly in sheet form, as substitutes for glass. They are advantageous in that they do not shatter or at least are more resistant than glass to shattering. Typical thermoplastics employed for this purpose are polycarbonates and poly(methyl methacrylate).
Because these thermoplastics are subject to surface scratching and abrasion, degrading their transparency, they are traditionally provided with a silicone top coat, hereinafter sometimes designated "hardcoat", to improve abrasion resistance and also weather resistance. Hardcoats are typically produced by flow coating or dip coating the thermoplastic article with a liquid alkylalkoxy silane, preferably with silica suspended therein, followed by thermal curing.
The adhesion of hardcoats to thermoplastic surfaces, especially of polycarbonate, is frequently deficient unless a primer is first applied to the thermoplastic surface. Typical primers are solutions of poly(methyl methacrylate) in organic solvents. Such solutions may also be applied by dip coating or flow coating, followed by removal of the solvent by air drying and in turn by hardcoat application. The thickness of the primer coating is typically about 0.2-1.0 micron.
It will be apparent that dip coating or flow coating of a thermoplastic sheet, generally conducted with the sheet held in a vertical position, will result in a primer or hardcoat layer with considerable variation in thickness from top to bottom. The coating liquid flows downward due to gravity, and therefore the coating thickness is substantially greater at the bottom of the sheet than at the top. Thus, a silicone hardcoat may have a thickness in the range of 3-4 microns at the top and 6-8 microns at the bottom of the coated sheet.
In regions in which the thickness of the hardcoat is less than about 5 microns, it is frequently found that adhesion to a poly(methyl methacrylate) primer is poor. For example, catastrophic failure may be observed in a crosshatch tape adhesion test (ASTM method D3359). On the other hand, adhesion of a hardcoat with a thickness of 5 microns or greater to the same primer may be adequate. Hardcoat adhesion is generally independent of the thickness of the primer layer, but variation in adhesion depending on hardcoat thickness is, of course, undesirable.